information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Salinity regime

Salinity is a measure of the concentration of dissolved salts in seawater. Salinity is defined as the ratio of the mass of dissolved material in sea water to the mass of sea water (UNESCO, 1985; TEOS-10, 2010). The term 'Absolute Salinity' (SA), measured as g/kg (mass fraction of salt in seawater) has been adopted as the standard SI unit for salinty, for use in calculations of the thermodynamic properties of seawater, by the International Oceanographic Commission (see TEOS-10, 2010). The term 'Practical Salinity (SP) , based on conductivity, is being phased out.

Unfortunately, salinity has been reported in numerous ways in the past, for example, as parts per thousand (ppt or ‰), as the 'practical salinity unit' (psu) or as 'salinity' without any units. Therefore, for the sake of accuracy when referring to salinity in our on-line reviews, the units used by the original authors are quoted in the text.

Salinity levels (adapted from Hiscock, 1996)
Full salinity 30-40
Variable salinity 18-40
Reduced salinity 18-30
Low salinity < 18
Unknown salinity ???


  • Hiscock, K. (ed.), 1996. Marine Nature Conservation Review: rationale and methods. Peterborough: Joint Nature Conservation Committee. [Coasts and seas of the United Kingdom. MNCR series.]
  • TEOS-10 Thermodynamic Equation of Seawater 2010.
  • UNESCO (United Nations Educational, Scientific and Cultural Organization), 1985. The International System of Units (SI) in Oceanography. Report of IAPSO working group on symbols, units and nomenclature in physical oceanography (SUN). IAPSO Publication Scientifique, no. 32, UNESCO technical papers in marine science, no. 45.